Introduction

In chemical changes, the molecules break apart, but the atoms never disappear. Keeping this straight requires an understanding of "atomic inventories".

Activity

For example, if you had two carbon dioxide molecules, it would look like this: 2(CO2). That is the same thing as CO2 + CO2 . In your “atom inventory”, you would have 2 carbon atoms and 4 oxygen atoms.

We put the "Before" and "After" molecules on opposite sides of an equal sign, and look to see if they balance. For example,

• C + O => CO2 is not a balanced equation, because you have more oxygen on the right side than you do on the left.
• C + O2 => CO2 is a balanced equation, because number of atoms matches up on both sides.